ENCN253 16S2: Soil Mechanics Assignment 1 [65marks] DUE: Friday July 29 at 2pm (in ENCN 253 homework boxes) Guidelines: Please write your solutions...

ENCN253 16S2: Soil Mechanics Assignment 1 Page 1 of 3 Lecturer: Gabriele Chiaro Assignment 1 [65marks] DUE: Friday July 29 at 2p m (in ENC N 25 3 homework boxes) Question 1 [5 marks] For a given soil show that : a) b) Question 2 [15 marks] The volumes of solids, water, and air of a quartz -domina ted sand with Gs=2.6 55 are 500 cm 3, 300 cm 3 and 200 cm 3, respectively . a) Draw a phase diagram for this soil. b) Determine the void ratio (e), porosity (n), specific volume ( v), degree of saturation ( Sr) and moisture content ( w)? c) Determine the bulk unit weight (γ) and dry unit weight ( γdry) of the soil? d) If the soil is compacted and half of the air is squeezed out of the voids, what would be the new void ratio and bulk unit weight? What is the percentage reduction in the total volume of the soil (relative to the o riginal volume)? Provide the new phase diagrams to support your answer e) How does the moisture content (w) of the compacted soil compare to that of the soil in its initial state? Provide calculations to support your answer. Question 3 [15 marks] The values of e min and e max for a pure silica sand ( Gs = 2.65 3) were found to be 0.6 32 and 0. 962, respectively. a) What is the corresponding range in dry density (in kg/m 3) and dry unit weight (in kN/m 3)? b) If the in -situ void ratio is 0.7 62 , what is the relative density? Give a term to classify the relative density. c) If 1 .2 kg of dry soil at the in -situ void ratio were then to be 73% saturated in a mould , what would be its total mass (in kg)? What would be its volume (in cm 3)? What would be the bulk unit weight (in kN/m 3)? d) If a vibration w as applied to the soil in the mould , via a vibrating table, so that the volume is reduced by 2.4 %, what would be the new void ratio, relative density and degree of saturation? Give a term to classify the new relative density. Guidelines:  A COVER SHEET (showing your name, ID number, course name and number, and date of submission) should be attached to the front of your solutions (homework submitted without a coversheet will receive a mark of zero).  Please write your solutions on CIVIL ENGINEERING CALCULATION PAPER (available from the ENSOC shop). The final answers to numerical questions should be wr itten clearly in the blank column on the right hand side of the page and underlined . Please write your solutions in a clear, neat, and professional manner.  Homework will be returned to the hand -back boxes once they have been marked (you will be notified in lectures / by email when they are available for pick up). Worked solutions will be provided on Learn (Moodle) shortly after this .  Homework should be submitted by the due date. Late submissions will not be accepted (see Course Outline). w e Se V W w r T s dry ) 1(      w dry s T T sat G V W              1 1 ENCN253 16S2: Soil Mechanics Assignment 1 Page 2 of 3 Lecturer: Gabriele Chiaro Question 4 [15 marks] The results of sieve analyses of three soils are shown below. H ydrometer ana lyses were also performed for soil s B and C, as more than 10 % of the soil passed through the 63μm sieve. Table: Percent finer (by weight ) Sieve Size / Particle Size A B C 20mm 100 100 - 10mm 95 98 - 6.3mm 88 96 - 2mm 23 93 - 600μm 0 86 - 425μm - 82 - 300μm - 77 100 150μm - 72 96 63μm - 68 82 40μm - 62 69 20μm - 37 43 6μm - 21 35 2μm - 10 21 Also, Atterberg limit tests undertaken on Soil C showed that the li quid limit w LL was 71% and plastic limit wPL was 3 0%. A sample of Soil C obtained from a borehole, tested for moisture content gave the following results: Mass of soil + container before placing in oven at 10 5ºC. 185.2 g Mass of soil + container after re moval from oven after 24 hrs. 125.6 g Mass of container 41.6 g (a) Plot a Particle Size Distribution (PSD) graph and d etermine the percent gravel, percent sand , percent silty and percent of clay for each soil: A, B and C. (b) Find the coefficient of unifor mity (Cu) and the coefficient of gradation (Cc) for each soil where possible . (c) Describe each of the soils: A, B and C using the NZ system (both the grading and the dominant particle sizes should be mentioned). (d) Calculate the plasticity index IP, the moisture content w and the liquidity index, IL for Soil C. (e) Present a qualitative description of soil C at its natural moisture content. Question 5 [15 marks] When a soil is used as a structural material or fill, it is generally compacted into place. The optimum wa ter content for compaction is often estimated in the lab by means o f a Standard Proct or Compaction test. Below are the results of 6 such tests carried out for a clayey silt: A B C D E F Moisture content w (%) 6 8 9 11 12 14 Dry density dry (kg/m 3) 150 8 1779 1888 1927 1886 1723 In the field, a “sand replacement test” may be carried out to determine the field unit -weight (i.e. how well the ground has been compacted compared to the theoretical value from the lab test). In this field test, a small hole i s excavated in the area where the soil has been compacted and this soil is weighed . A sample is taken for moisture content determination. A known mass of dry standard sand is then pored into the hole.

This sand has an approximately known dry density when g ently pored in this way , so from this and knowing the mass of dry sand that replaces the compacted soil, along with the mass and moisture content of the compacted soil, the dry density of the compacted material can be estimated. ENCN253 16S2: Soil Mechanics Assignment 1 Page 3 of 3 Lecturer: Gabriele Chiaro For this field test, the f ollowing was noted: • Calibrated dry density of Standard Sand = 1 610 kg/m 3 • Mass of sand used to fill hole = 2 165 g • Mass of compacted soil excavated from hole = 30 35 g • Moisture content of compacted soil = 1 0.3 % a) Plot the dry unit weight (kN/m 3) again st the water content for the six samples and use your graph to estimate the optimum water content for compaction in the lab . b) Determine the dry density of compaction in the field from the sand replacement test. c) What is then the “relative compaction” – i.e. the ratio between the field dry density and dry density obtainable at optimum moisture content from the Standard Compaction test ? d) Why is the dry density of the soil used to assess the degree of compaction, rather than the bulk density?